Is it safe breastfeeding when taking mesalamine?

La Lechea League leaders have access to information that gives drug tiers and half life's in regards to breastfeeding. I would give on a call for that information. If you don't know a local group, you can look up the info on their website

Congratulations on the little one!
My guess is hat your GI has had this question asked more often than your baby’s Pediatrician. I would reach out to some breastfeeding experts like la Leche or a lactation consultant who is familiar with mothers who have chronic health conditions. There are databases online which track the safety of breastfeeding while in medication, this is what I found quickly:
https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/f?./temp/~Jx0IGA:1

There have been some moms who have breastfed with UC, I didn’t have UC when my boys were little, so I can’t speak to personal experience. Hopefully some mom’s who have done it will add their two cents!

6. Lactation

6.1. Influence of lactation on disease activity and
child outcome

Women with IBD are as likely as the general population to breastfeed their infants; the mean duration of the breastfeeding period is shorter than in the general population.196,197 Breastfeeding is not associated with
an increased risk of disease flare and may even provide a protective effect against disease flare in the postpartum year.70,198–201 However, in
one study, lactation was associated with an increase in disease activity; but medication cessation was a relevant confounding factor.202
It is reported that breastfeeding might be protective against the development of early onset of IBD in children.201 Thus, breastfeeding is suggested when possible, as it may have a protective effect against
development of IBD in the offspring; however, the quality of existing data is generally poor. These findings need to be investigated in well-designed prospective studies.
There is also limited evidence that breastfeeding may be associated with a more beneficial gut microbe profile, as it seems to be associated with highest numbers of bifidobacteria and lowest numbers of C. difficile and E. coli,203,204 which may help protect against the development of IBD. Formula milk feeding or breastfeeding do not seem to differ in terms of onset of intestinal inflammation,205 but controversial data exist.206

6.2. Medical treatment during lactation

ECCO Statement 6B

5-ASA derivatives [EL3], thiopurines [EL3], anti-TNFs
[EL4], and corticosteroids [EL4] are of low risk for breast-
fed infants

For a summary of risks of frequent IBD medications used during lactation, see Table 1.
Sulfasalazine is of low risk during breastfeeding. The sulfapyridine moiety is absorbed in minimal amounts and is excreted in milk, but the milk/serum ratio is acceptable.207 This low risk of aminosalicylates has been confirmed in prospective trials.208

Table 1. ECCO overview on drug risk during pregnancy and lacatation.
Drug............... During pregnancy.......During lactation
Mesalazine......Low risk......................Low risk
Sulfasalazine...Low risk......................Low risk

Source: The Second European Evidenced-Based Consensus on Reproduction and Pregnancy in Inflammatory Bowel Disease

ecco-jcc.oxfordjournals.org/content/eccojc/9/2/107.full.pdf